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10 Top Telescope Questions

Like everyone who writes about backyard astronomy, I get my share of letters. Some of them are generated by things I’ve written in magazines, and others come from my own website. One message that comes through loud and clear is that telescope ownership is fraught with all kinds of potential complications. Even the basics of aiming and focusing a scope can cause confusion if you’re new enough to the hobby. In fact, some of the most common questions are the “easy” ones that often don’t get answered in the owner’s manual  and these days some scopes don’t even come with a manual!

So gathered together here are the 10 questions I hear most often  along with my answers. If you’re new to using a telescope (or even if you’ve been around the block a few times), you’ll probably find some issues here that you’ve wondered about.

Ed Ting’s first telescope was a 6-inch reflector that he saved for by working at Burger King. Find lots more of his advice and opinions at his scopereviews.com website.

Eyepieces come in many sizes and designs to suit all tastes and budgets. Like everything in life, you get what you pay for, but designs with desirable qualities can be found at reasonable cost once you know what to look for.

10. How do I figure out the magnification of my telescope?

The easiest way is look in the scope’s manual. The magnification of your telescope will almost certainly be printed somewhere in the instructions. But if your scope didn’t come with a manual, or if you’ve gotten a new eyepiece, you can run the numbers yourself.

Simply take the focal length of your telescope’s objective (its main mirror or lens) and divide it by the focal length of your eyepiece. For example, if your telescope has a 1,200-mm focal length and you’re using a 25-mm eyepiece, the magnification is 48× (1,200 ÷ 25). To change the magnification or “power,” change eyepieces: put in a 10-mm eyepiece, and the magnification becomes 120× (1,200 ÷ 10).

If you want to calculate them right now, here's a handy calculator:

Telescope
focal length =

Eyepiece
focal length =

Magnification
=

Usually the focal length of your objective is printed on a sticker somewhere on the telescope (look for FL or F), while the focal length of the eyepiece is marked on its barrel, near the lens you look through. You’ll find both values given in millimeters (mm).

Owners of reflectors and Schmidt-Cassegrains periodically need to collimate (align) their optics to get the best images. Unfortunately, collimation is one of those things that’s much easier to do than to describe without the benefit of step-by-step pictures. But why reinvent the wheel here? An excellent, well-illustrated article on the subject has already been written by Nils Olof Carlin.

Left: A telescope on an altazimuth mount, like this popular Dobsonian design, simply moves up-down and left-right. Right: A telescope on an equatorial mount moves in the directions of celestial north-south and east-west. This makes it easier to follow the moving stars as the Earth turns, and it can also help you stay oriented when you use a star chart at the eyepiece.

Sky & Telescope diagram.

8. What’s an equatorial mount, and do I need one?

For those of us in the Northern Hemisphere, the stars appear to pivot around a point in the sky very near Polaris, the North Star. An equatorial (“EQ”) mount has one of its two axes aimed toward that point, and that allows you to follow a star or planet in your telescope’s eyepiece with just a single east-to-west motion. Add a small motor, and the telescope will track the stars automatically.

In contrast, telescopes equipped with altitude-azimuth (“alt-az”) mounts, including Dobsonian reflectors, require you to move your telescope with both up-down and left-right motions to accomplish the same thing.

When beginners first learn about mounts, they tend to conclude that they need an equatorial mount. Sure, these can be handy, but be aware they tend to be larger, heavier, and more expensive than a mechanically simpler alt-az mount. Also, you have to be prepared to spend some time learning how to use an equatorial mount  its motions aren’t quite as intuitive as those of an alt-az. An equatorial mount that’s been set up incorrectly (something I see all the time) is maddening to use and will actually hinder your tracking ability.

7. How come the objects in my telescope look so small/dim/featureless?

This question has many possible answers. Perhaps you’ve developed unrealistic expectations by looking at all the spectacular photos coming from professional observatories, the Hubble Space Telescope, and other sources. Sadly, you just can’t see this kind of detail and color in a backyard telescope. If you’re looking at “deep-sky” objects like galaxies or nebulas, most often you’ll see a faint haze or smudge against a black background. A bigger scope will improve the view of faint targets  but you still won’t see bright colors.

Left: We're accustomed to beautiful color images of the Orion Nebula (M42). But most deep-sky objects manifest themselves in shades of gray at the eyepiece (right).

Through a backyard telescope Saturn or Jupiter will look no bigger than a bright dime in the middle of a black dinner plate. If your views of the planets don’t have the detail you expect, you could be falling victim to a number of ills. The usual culprits are an unsteady atmosphere, a telescope that hasn’t had enough time to cool down to the outside air temperature, or optics that aren’t big enough or good enough to deliver the kinds of features you’re hoping to see. And it could be a combination of all three!

Keep in mind that “seeing” while at the eyepiece is an art as well as a skill. The more you observe, the more you’ll see.

When cleaning lenses, apply fluid to your cleaning pad, not the lens itself. You donâ€™t want any fluid to seep in around the lensâ€™s edges; it may carry dissolved gunk to glass surfaces inside and leave stains where you canâ€™t reach them.

Craig Michael Utter

6. My mirror/lens/eyepiece is dirty. Should I clean it, and with what?

There are two schools of thought on this  think of their practitioners as the Felix Ungers and Oscar Madisons of the telescope world.

A few years ago, at a large star party, the owner of a very expensive refractor brought his prized telescope to the manufacturer’s tent, asking him to clean its lens. The optician disappeared into the back. “Oh, boy,” the excited owner thought to himself. “I’m about to get a cleaning lesson from the master himself!” The optician came back a few minutes later with a bottle of Windex and an old, yellowed piece of cheesecloth. “You guys worry about this too much,” he said, as he scrubbed the lens in front of the horrified owner.

I know lots of people who, like this guy, fret over every speck of dust on their optics and obsessively clean them. At the other end of the spectrum are people  and I used to be one of them  who never clean their optics. Their rationale is that you can possibly do more damage to your precision glass surfaces by cleaning them than by just letting them be dusty.

The right approach is probably somewhere in between these extremes. The key is prevention and moderation  use those dust caps, and clean only when absolutely necessary.

To clean your reflector’s mirror, take it out of its support cell and run distilled water over its surface. If the mirror looks OK after this, stop  you’re done. Carefully set the mirror on edge and let it air dry. If not, let the mirror soak in warm water with a tiny drop of dishwashing detergent added to it. (Make sure your sink is clean first!) Follow with a rinse of distilled water, and you should be good to go. If it’s still dirty, more drastic measures may be necessary. For the gory details, check out Alan MacRobert’s article on cleaning optics.

When it comes to grubby refractor lenses or dirty eyepieces, a different approach is needed. Whatever you do, don’t dismantle them. The best way to clean lenses is to gently wipe away the grime with a lint-free cloth or cotton swap moistened with pure isopropyl alcohol or a specialized lens-cleaning solution. But before you start, make sure there are no dust particles on the surface of the lens; a blast of air or a feather-light brushing will usually take care of this. A heavier-handed approach might scratch the coatings or perhaps even the glass itself. Refer to the above article for more suggestions.

Three ways to take aim at the sky. Left: Lensless peep sights suffice for small telescopes with wide fields of view. Center:/ Reflex sights project a dim red dot or circle on the sky, improving precision. Right: Finderscopes make more targets visible and enable the most precise pointing. But many models turn the sky upside down or are hard to use with eyeglasses on.

Sky & Telescope / Chuck Baker

5. What’s a “Telrad,” and why do I need one if my telescope already has a finder?

Until recently, all finders were themselves miniature telescopes. Today, more and more scopes are sold with so-called “unit-power” (1×) or “red-dot” finders. The Telrad was the first commercial finder of this kind, but many others have since appeared on the market.

Using one couldn’t be easier. Switch it on, and a glowing red dot or bull’s-eye target appears projected on the sky when you look through a little window. To aim your scope, all you have to do is move the tube until that the red dot sits on your desired sky object.

Observers often find that red-dot finders and traditional finderscopes complement each other  it doesn’t have to be an either/or situation. I find it convenient to use a unit-power finder to get the telescope pointed in the general area, and then zero in on my target with my normal finderscope.

The view in your telescope may be either right-side up (left column) or upside down (right column); this is no big deal. And it may be either correct (top row) or mirror-imaged (bottom row), which is more of a problem. Here are the four possible combinations.

4. Help! The Moon looks upside down in my telescope!

Yes, that happens. Or, in some telescopes, the Moon looks right-side up but mirror-reversed (flipped left to right). Don’t worry, nothing is broken  this is perfectly normal. Since there’s no “up” or “down” in space, most stargazers just accept this optical quirkiness and don’t let it bother them. And, besides, you’ve seen this kind of thing before. After all, when you look at yourself in a mirror, isn’t the image staring back at you flipped left-right?

As a general rule, if your telescope’s optics have an even number of reflections (as is the case for Newtonian reflectors, like a Dobsonian), you’ll get an upside-down view. Scopes that have an odd number of reflections (refractors or compound telescopes, used with a 90° star diagonal) give a mirror-reversed view. If you’re not sure which case applies to you, just aim your scope at the Moon or a building and compare the view with what you see in binoculars or with your eyes alone. You’ll figure out right away what’s going on.

A refractor like this might be perfect for you, but maybe not.

Sky & Telescope / Craig Michael Utter

3. Which is better: a refractor or a reflector?

This used to be an easier question to answer. My old advice was: use a refractor for sharp images of the Moon, planets, and double stars, and use a big reflector to hunt down dim, distant deep-sky objects.

Today, there are more kinds of scopes  and the distinctions between them have blurred a bit. Refractors are getting larger (though they’re not cheap), while some of the best views of Saturn and Jupiter I’ve ever had were through newer, high-quality reflectors. And compound telescopes use both mirrors and lenses.

So these days I suggest that it’s important to match the scope with your budget and what you plan to observe most of the time. But that old adage is still generally true: good refractors are still hard to beat when viewing the Moon and planets, and if you want to search for dim objects, then a large reflector is still the best choice.

2. I have $500 to spend. Should I get a small, computer-controlled telescope, or a big telescope with no electronics at all?

There is no right answer to this question, just opinions  and here’s mine. In the lower price range, I usually recommend keeping things simple. You can’t have everything for $500, so spend your money on good optics and a simple, sturdy mount. In that price range, I usually recommend a 6- or 8-inch Dobsonian reflector. If you have decided that you simply must have a computer-controlled telescope, try to up your budget to around $800. “Go To” scopes in this price range tend to be more robust, reliable, and accurate than cheaper ones.

1. I bought a 675× telescope at my local X-Mart for $69. I can’t get it to work. Can you help?

At last, here’s the question I get asked most often!

Inexpensive department-store telescopes are still the bane of our hobby. The same holds true for closeout telescopes from mail-order catalogs, the ones sold in sporting-goods and toy stores, and most scopes sold on eBay. Some of these are so cheaply made that they may not show you anything worth looking at.

As a rule of thumb, avoid telescopes advertised to deliver unrealistically high magnifications (above 500×). Their high-power views will be dim and blurry  that’s if you can even get the scope aimed at anything. Also, most telescopes under $200 should be regarded with skepticism. Although a few good-quality exceptions do exist near this price, you can avoid most junky scopes by simply setting your budget higher. And sometimes you can spot a lousy scope just by looking at it. If it looks cheaply made, chances are it is.

If you’ve already purchased one of these so-called “telescopes,” see if you can return it  even if you have to pay a restocking fee. Then go to a specialty retailer and chalk up the money you lost as the cost of a little education. A reputable, knowledgeable telescope dealer will sell you something that fits your needs. The kicker is that a “legitimate” good-quality telescope may actually cost less than the one you saw down at the mall.

One thought on “10 Top Telescope Questions”

I am a 77 yr old male in good shape. I would like you to recommend a good go to telescope that doesn’t take a rocket scientist. I have seen a refractor advertised that you only have to point at one star and will align itself. Some thing in the $1500.00 to $2000.00 range that is simple to set up. Thanks for your time.

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